1. Field of the Invention
The present invention relates to regenerative processes for removal of sulfur dioxide from gases which employ an aqueous absorbent containing sodium or potassium carbonate to convert sulfur dioxide to sulfite. The term "carbonate" as used herein referes to carbonate and bicarobonate, jointly and severally; and the term "sulfite" similarly refers to sulfite and bisulfite, jointly and severally.
2. Description of the Prior Art
The present invention is a proposed improvement in those regenerative processes for removing SO.sub.2 from SO.sub.2 -containing gases which convert the SO.sub.2 in a scrubbing circuit first to sulfite by means of an absorbent and then the sulfite to a thiosulfate by means of a sulfite-reducing agent in a recirculating aqueous solution of thiosulfate. The absorbent is sodium or potassium carbonate and the sulfite-reducing agent is a sulfide or mixture of sulfides of the same cation as the absorbent. The concentration of thiosulfate, also of the same cation as the absorbent, is maintained substantially constant by withdrawal from the recirculating aqueous solution of a minor portion which corresponds to the net make of thiosulfate. The withdrawn portion is subjected to one or more regenerative steps to produce a mixture of carbonate and sulfide for return in the proper amounts to the recirculating solution of thiosulfate for reuse therein as absorbent and sulfite reducing agent respectively.
More particularly, the present invention is a proposed improvement in the regeneration system of the process described and claimed in U.S. Pat. No. 3,937,787. The process covered by said patent comprises the following absorption and regeneration operations:
(1) maintaining in a scrubbing circuit a recirculating aqueous solution which contains sodium or potassium thiosulfate in a concentration of at least 10 percent by weight of the solution;
(2) maintaining a gas scrubbing zone in said scrubbing cirucit wherein the SO.sub.2 -containing gas passes in intimate contact with sodium or potassium carbonate under sulfite-forming conditions to thereby convert the SO.sub.2 to sulfite;
(3) regulating the amount of sulfite in the recirculating aqueous solution so that its concentration is always less than 5 percent by weight thereof;
(4) converting the sulfite formed in the scrubbing zone to thiosulfate while in the scrubbing circuit but in a sulfite-reduction zone separate from the scrubbing zone, by means of a sulfide or mixture of sulfides;
(5) withdrawing a minor portion or slip stream of the recirculating aqueous solution from the scrubbing circuit to a regeneration system wherein thiosulfate contained in the slip stream is converted to a mixture of carbonate and sulfide; and
(6) returning to the scrubbing circuit a regenerated aqueous stream containing in admixture the sulfide and carbonate in the proper ratio.
The proper ratio (designated R and sometimes called "Acceptability Ratio") of sulfide and carbonate in the regenerated aqueous solution is defined as follows: ##EQU1## where (S.degree.)=gram atoms sulfur with valence number zero per 100 grams solution;
(S.sup.-2)=gram atoms sulfur with valence number equal to -2 per 100 grams solution; PA1 .SIGMA.M=gram atoms of M per 100 grams solution persent in said aqueous solution as MHS, M.sub.2 S, M.sub.2 S.sub.x, M.sub.2 CO.sub.3, MHCO.sub.3, MOH; and PA1 M=Na or K PA1 P.sub.(CO.sbsb.2.sub.) =partial pressure of CO.sub.2 (atm) PA1 P.sub.(H.sbsb.2.sub.S) =partial pressure of H.sub.2 S (atm)
In the foregoing definition of the Acceptability Ratio, the cation M fed to the system as MOH and/or M.sub.2 CO.sub.3 to make up the loss of M from the system is included.
The conversion of thiosulfate to the desired mixture of sulfide and carbonate may be effected, as taught in the above-cited U.S. Pat. No. 3,937,787, by reaction of the thiosulfate with a reducing gas containing CO as the principal reductant. To the best of my knowledge, this reaction, although demonstrated in laboratory and pilot plant, has not yet been used commercially. A process which is presently used in the paper industry is the so-called molten salt sulfate reduction process. Because of its proven commercial feasibility, it is desirable to adapt it, if possible, to the conversion of thiosulfate to a mixture of sulfide and carbonate of desired relative proportions for use in the SO.sub.2 removal processes described above.